Process to produce linear pentenes and metathesis thereof

ABSTRACT

Mixed pentenes may be converted to propylene by feeding an alcohol, linear pentenes, and isopentenes to an etherification reactor. The alcohol and isopentenes may be reacted in the etherification reactor to convert isopentenes to tertiary amyl alkyl ether, which may be separated from the linear pentenes, recovered as a linear pentene fraction. The tertiary amyl alkyl ether may be fed to a decomposition reactor to convert at least a portion of the tertiary amyl alkyl ether to alcohol and isopentenes. The alcohol and isopentenes may then be separated to recover an isopentene fraction and an alcohol fraction. The isopentene fraction is then fed to a skeletal isomerization reactor to convert at least a portion of the isopentenes to linear pentenes, the effluent from which may be recycled to the etherification reactor. Ethylene and the linear pentene fraction may then be to a metathesis reactor to produce propylene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S.Provisional Application Ser. No. 61/728,046, filed Nov. 19, 2012, whichis herein incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

Embodiments disclosed herein relate to production of propylene viametathesis of pentenes with ethylene. More specifically, embodimentsdisclosed herein relate to the efficient separation and conversion ofisopentenes in a mixed pentene feed (isopentenes and linear pentenes) tolinear pentenes and metathesis of the linear pentenes with ethylene toform propylene.

BACKGROUND

The high demand of polypropylene is increasing. This propylene growthcombined with the scarcity of butenes (for making propylene bymetathesis) has created a market for the utilization of pentenes to makepropylene by metathesis. C5 olefins are available from two main sources.They are (i) C5 olefins available from stream cracker, and (ii) C5olefins available from refinery, both containing significant amounts ofisopentene along with the linear pentenes.

When linear pentenes are fed to a conventional metathesis reactor, thefollowing reactions may occur:

-   -   (a) 1-pentene→2-pentene (Isomerization);    -   (b) 2-pentene+ethylene-→1-butene+propylene (Metathesis);    -   (c) 1-butene-→2-butene (Isomerization);    -   (d) 2-butene+ethylene-→2 propylene (Metathesis).        1-Pentene is isomerized to 2-pentene. The metathesis reaction of        1-pentene with ethylene is non-productive (products are same as        reactants). The overall linear C5 olefin reaction can thus be        shown as:    -   1 linear pentene+2 ethylene-→3 propylene.

When isopentenes are fed to a conventional metathesis reactor, thefollowing reactions may occur:

-   -   2-methyl-2-butene+ethylene-→isobutene+propylene (Metathesis)    -   2-methyl-1-butene-→2-methyl-2-butene (Isomerization)    -   3-methyl-1-butene-→2-methyl-2-butene (Isomerization)        The reactions of 2-methyl-1-butene and 3-methyl-1-butene with        ethylene are non-productive. Thus, the metathesis of isopentenes        results in only one mole of propylene, one-third the        productivity of the linear pentene metathesis.

Typically, isopentenes content in these C5 streams is in the range of40-60 wt % and is the most abundant species. As shown in the reactionsabove, the processing of isopentenes by metathesis results in theinefficient conversion of pentenes to propylene. Only one mole ofpropylene is formed from every mole of isopentene, as opposed to threemoles of propylene from every mole of linear pentene.

Table 1 shows the boiling points of the linear pentenes and theisopentenes.

TABLE 1 Boiling points of Pentenes Boiling Point Pentene (° C.) Type3-Methyl 1-Butene 26.00 Isopentene 1-Pentene 29.95 Linear Pentene2-Methyl 1-Butene 31.20 Isopentene t-2-Pentene 36.30 Linear Pentenec-2-Pentene 36.90 Linear Pentene 2-Methyl 2-Butene 38.55 Isopentene

As can be seen from the staggered boiling points of the linear pentenesand the isopentenes, a simple separation of the linear pentenes fromisopentenes by distillation is not easily achievable.

SUMMARY OF THE DISCLOSURE

In one aspect, embodiments disclosed herein relate to a process forconverting mixed pentenes to propylene. The process may include: feedingan alcohol and a hydrocarbon stream containing linear pentenes andisopentenes to an etherification reactor; reacting the alcohol andisopentenes in the etherification reactor to convert at least a portionof the isopentenes to tertiary amyl alkyl ether; separating the linearpentenes from the tertiary amyl alkyl ether to recover a linear pentenefraction and a tertiary amyl alkyl ether fraction; feeding the tertiaryamyl alkyl ether fraction to a decomposition reactor; reacting thetertiary amyl alkyl ether in the decomposition reactor to convert atleast a portion of the tertiary amyl alkyl ether to alcohol andisopentenes; separating the alcohol and the isopentenes produced in thedecomposition reactor to recover an isopentene fraction and an alcoholfraction; feeding the isopentene fraction to a skeletal isomerizationreactor to convert at least a portion of the isopentenes to linearpentenes; recovering an effluent from the skeletal isomerization reactorcomprising isopentenes and linear pentenes; recycling the effluent fromthe skeletal isomerization reactor to the etherification reactor; andfeeding ethylene and the linear pentene fraction to a metathesis reactorto convert at least a portion of the linear pentenes and ethylene topropylene.

In another aspect, embodiments disclosed herein relate to a system forconverting mixed pentenes to propylene. The system may include: anetherification reactor for converting an alcohol and isopentenes totertiary amyl alkyl ether; a separator for separating the linearpentenes from the tertiary amyl alkyl ether and to recover a linearpentene fraction and a tertiary amyl alkyl ether fraction; adecomposition reactor for reacting for converting at least a portion ofthe tertiary amyl alkyl ether in the tertiary amyl ether fraction toalcohol and isopentenes; a separator for separating the alcohol and theisopentenes produced in the decomposition reactor and to recover anisopentene fraction and an alcohol fraction; a skeletal isomerizationreactor to convert at least a portion of the isopentenes in theisopentene fraction to linear pentenes; a flow line for recovering aneffluent from the skeletal isomerization reactor comprising isopentenesand linear pentenes and recycling the effluent from the skeletalisomerization reactor to the etherification reactor; and a metathesisreactor for reacting at least a portion of the linear pentenes in thelinear pentene fraction with ethylene to form propylene.

Other aspects and advantages will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified block flow diagram of a process for producingpropylene from mixed pentenes according to embodiments disclosed herein.

FIG. 2 is a simplified process flow diagram of a separation schemeuseful in embodiments disclosed herein.

FIG. 3 is a simplified process flow diagram of a process for producingpropylene from mixed pentenes according to embodiments disclosed herein.

FIG. 4 is a simplified process flow diagram of a process for producingpropylene from mixed pentenes according to embodiments disclosed herein.

DETAILED DESCRIPTION

In one aspect, embodiments disclosed herein relate to production ofpropylene via metathesis of pentenes with ethylene. More specifically,embodiments disclosed herein relate to the efficient separation andconversion of isopentenes in a mixed pentene feed (isopentenes andlinear pentenes) to linear pentenes and metathesis of the linearpentenes with ethylene to form propylene.

To separate the isopentenes from linear pentenes, the mixed pentene feedmay be fed to an etherification reactor to react at least a portion ofthe isopentenes with an alcohol, such as a C1 to C4 alcohol, to producetertiary amyl alkyl ethers. Linear pentens do not react to anysubstantial degree in the etherification reactor. The tertiary amylalkyl ethers, having a significantly different boiling point than thelinear pentenes, may then be separated from the linear pentenes toproduce a linear pentene fraction and a tertiary amyl alkyl etherfraction. The linear pentene fraction may then be fed to a metathesisreactor for conversion of the linear pentenes to propylene viametathesis with ethylene, producing up to 3 moles of propylene per moleof linear pentenes.

The tertiary amyl alkyl ether fraction may then be fed to adecomposition reactor to convert the tertiary amyl alkyl ether back intothe alcohol and the isopentene. The reaction products are separated, andthe alcohol may be recycled back to the etherification reactor (systemmay be essentially closed loop with respect to the alcohol). Thedecomposition reaction may be performed in one or more fixed bedreactors in the presence of a catalyst. Addition of water or a tertiaryalcohol (such as tertiary amyl ether, used as a water equivalent,decomposing to an isoolefin and water) to the tertiary alkyl ether feedstream, for example, may be beneficial to suppress unwanted sidereactions. Other water equivalents may additionally be used.

The isopentenes produced in the decomposition reactor may then be fed toa skeletal isomerization reactor to convert a portion of the isopentenesto linear pentenes. Skeletal isomerization is a reversible reaction, andat a temperature of about 400° C., the skeletal isomerization reactoreffluent may contain about 65% to 69% isopentenes (other temperaturesmay result in a different equilibrium mixture). As such, a highconcentration isopentene feed is beneficial, allowing for maximumshifting of the isopentenes to linear pentenes. The high concentrationisopentene feed is provided by the decomposition reactor, which mayprovide for an essentially pure isopentene feed in some embodiments;greater than 90% isopentene in other embodiments; and greater than 80%isopentene in other embodiments.

The mixed pentenes recovered from the skeletal isomerization reactor maythen be recycled back to the etherification reactor, for conversion andseparation of isopentenes and forwarding of the additional linearpentenes produced via skeletal isomerization to the metathesis reactor.

Mixed pentene feedstocks useful in embodiments disclosed herein mayinclude linear pentenes and isopentenes. Mixed pentene feedstocks mayalso include various other hydrocarbon components, including C4 to C6paraffins and olefins. In some embodiments, the mixed pentene feedstockmay be a C5 hydrocarbon fraction from a catalytic or steam cracker,where the C5 fraction may include linear pentenes, isopentene,n-pentanes, and isopentanes. In some embodiments, the mixed pentenefeedstock may include isopentene at a concentration of greater than 40mol %, 50 mol %, or 60 mol %.

Alcohols useful in the etherification reactor may include C1 to C4alcohols, among others. For example, the alcohol feed to theetherification reactor may include at least one of methanol, ethanol,n-propanol, n-butanol. In some embodiments, the alcohol feed ismethanol.

FIG. 1 illustrates a simplified block flow diagram of a process forproducing propylene from mixed pentenes according to embodimentsdisclosed herein. Mixed pentenes and an alcohol, such as methanol, maybe fed via flow lines 10 and 12, respectively, to etherificationreaction zone 14. Etherification reaction zone 14 may include one ormore etherification reactors containing an etherification catalyst. Theisopentenes in the mixed pentenes may then be reacted with the alcoholat appropriate reaction conditions over the etherification catalyst toproduce a reaction effluent including tertiary amyl alkyl ethers andlinear pentenes. The linear pentenes and tertiary amyl alkyl ethers inthe reaction effluent are then separated to form a linear pentenefraction and a tertiary amyl ether fraction.

The tertiary amyl alkyl ether fraction is fed via flow line 16 todecomposition reaction zone 18, which may include one or more reactorscontaining a decomposition catalyst. The tertiary amyl alkyl ether isthen contacted with the decomposition catalyst at appropriate reactionconditions to crack the tertiary amyl alkyl ether into the constituentcomponents, namely the alcohol and isopentene. The reaction products areseparated into an alcohol faction, which may be recycled back to theetherification reactor via flow line 20, and an isopentene fraction.

The resulting isopentene fraction, which may be a high purity isopentenefraction (i.e., greater than 90%, 95%, 98%, or even 99% isopentene) maythen be fed via flow line 22 to skeletal isomerization reaction zone 24.Skeletal isomerization reaction zone 24 may include one or more reactorscontaining a skeletal isomerization catalyst. The isopentene is thencontacted with the skeletal isomerization catalyst at appropriatereaction conditions to convert at least a portion of the isopentene tolinear pentenes. The reaction effluent from the skeletal isomerizationreaction zone 24, including both linear and iso-pentenes, may then berecycled via flow line 25 to the etherification reaction zone 14, forcontinued reaction and separation of the isopentenes from the linearpentenes.

The linear pentene fraction recovered from etherification reaction zone14 via flow line 26, including linear pentenes as the primary olefiniccomponent, and ethylene feed 28 may be forwarded to metathesis reactionzone 30. Metathesis reaction zone 30 may include one or more metathesisreactors containing a metathesis catalyst. The linear pentenes andethylene are then contacted with the metathesis catalyst at appropriatereaction conditions to convert at least a portion of the linear pentenesand ethylene to propylene.

The metathesis reaction products, including unreacted ethylene,propylene, butenes, and unreacted pentenes may then be recovered viaflow line 32 and forwarded to separation zone 34. Separation zone 34includes one or more distillation columns and/or extractive distillationcolumns for separating the metathesis reactor effluent into variousdesired fractions, which may include ethylene fraction 36, propylenefraction 38, butene/pentene fraction 40, and heavies fraction 42. Ifdesired, ethylene fraction 36 and butene/pentene fraction 40 may berecycled to metathesis reaction zone 30 for continued production ofpropylene.

Catalysts useful in the etherification reactor may include any catalysttypically used in etherification processes, such as conventional cationexchange resins and/or zeolites. Examples of etherification catalystsuseful in embodiments disclosed herein are described in, for example,U.S. Pat. No. 7,553,995, which is incorporated herein by reference.Other suitable etherification catalysts are described in, for example,U.S. Pat. Nos. 5,190,730, 5,231,234, 5,248,836, 5,292,964, 5,637,777,and 6,107,526, among others.

Operating conditions in the etherification reactor may vary based on thefeed mixture, reactor type, reactor/reaction phase(s), catalyst type,and other variables known to those skilled in the art. Etherificationreaction conditions may include, for example, a temperature in the rangefrom about 30° C. to about 150° C. in some embodiments; from 50° C. to120° C. in other embodiments; and from 80° C. to 110° C. in yet otherembodiments, where the pressure may range from 0.5 bar to 10 bar in someembodiments, and from 1 bar to 5 bar in other embodiments.

Catalysts useful in the decomposition reactor may include anydecomposition catalyst, which may include various supported andunsupported acid catalysts. Acid catalysts that may be used according toembodiments herein may include solid acid catalysts, natural clays,synthetic clays, zeolites or molecular sieves, acid resin catalysts,such as solfonic acid resins or acid cation exchange resins, and othersas known to those skilled in the art. In some embodiments, thedecomposition catalyst may include an HF treated amorphous syntheticalumina-silica catalyst or a selectively poisoned HF treated amorphoussynthetic alumina-silica catalyst, such as disclosed in U.S. patentapplication Ser. No. 12/260,729, which is incorporated herein byreference.

Operating conditions in the decomposition reactor may vary based on thefeed mixture, reactor type, catalyst type, reactor/reaction phase(s),and other variables known to those skilled in the art. Decompositiontemperatures may range from 100° C. to 500° C. in some embodiments; from130 to 350° C. in other embodiments and from 150° C. to 300° C. in yetother embodiments. The decomposition reaction may be carried out underpressures in the range from 1 to 22 bar (0 to 300 psig) in someembodiments; from 1 to 11 bar (0 to 150 psig) in other embodiments. Insome embodiments, the pressure is maintained such that the productolefin is in the liquid phase or partially in the liquid phase at thereaction temperature used. The liquid hourly space velocity (LHSV) (thevolume of liquid per volume of catalyst per hour) at which the reactionis carried out may be within the range from 0.5 to 200 h⁻¹ in someembodiments; from 1 to 50 h⁻¹ in other embodiments; and from 1 to 10 h⁻¹in yet other embodiments.

Catalysts useful in the skeletal isomerization reactor may includepromoted acidic catalysts. In some embodiments, the skeletalisomerization catalyst may include a molecular sieve or zeolitecatalyst, such as a silica/alumina phosphate (SAPO), ZSM-22, ZSM-23, orvarious molecular sieves having a 10-membered ring structure, such asdisclosed in EP0703888B1. Various metallosilicate skeletal isomerizationcatalysts are disclosed in, for example, U.S. Pat. No. 5,019,661. Othercatalysts that may be used for skeletal isomerization disclosed hereininclude those disclosed in, for example, U.S. Pat. No. 4,650,917.

Operating conditions in the skeletal isomerization reactor may varybased on the feed mixture, reactor type, catalyst type, reactor/reactionphase(s), and other variables known to those skilled in the art.Skeletal isomerization reaction temperatures may range from 50° C. to500° C. in some embodiments; from 125 to 400° C. in other embodimentsand from 150° C. to 350° C. in yet other embodiments. The skeletalisomerization reaction may be carried out under pressures in the rangefrom 1 to 50 bar in some embodiments; from 2 to 40 bar in otherembodiments. The olefin-containing feed may be supplied to the catalystat a weight hourly space velocity (WHSV) in the range from about 0.1 toabout 100 h⁻¹, and with a hydrogen partial pressure, when used, in therange from about 0.1 to 30 bar. As competing hydrogenation reactions mayoccur, depending upon hydrogen partial pressures and temperatures used,as well as the fact that the skeletal isomerization reaction isreversible, typically achieving equilibrium in the reactor, where theequilibrium point may be temperature dependent, care should be takenwhen selecting the appropriate reaction conditions.

Catalysts useful in the metathesis reactor may include any knownmetathesis catalyst, including oxides of Group VIA and Group VIIA metalson supports. Catalyst supports can be of any type and could includealumina, silica, mixtures thereof, zirconia, and zeolites. In additionto the metathesis catalyst, the catalyst contained in the metathesisreactor may include a double bond isomerization catalyst such asmagnesium oxide or calcium oxide, for converting 1-butene and 1-penteneto 2-butene and 2-pentene, allowing for increased production ofpropylene via metathesis with ethylene. In some embodiments, thecatalyst may include a promoter to reduce acidity; for example, analkali metal (sodium, potassium or lithium), cesium, a rare earth, etc.In some embodiments, the metathesis or mixed metathesis/double bondisomerization catalyst may include those described in US20110021858 orUS20100056839, for example.

The metathesis reactor may operate at a pressure between 1 and 40 barinsome embodiments, and between 5 and 15 bar in other embodiments. Themetathesis reactor may be operated such that the reaction temperature iswithin the range from about 50° C. to about 600° C.; within the rangefrom about 200° C. to about 450° C. in other embodiments; and from about250° C. to about 400° C. in yet other embodiments. The metathesisreaction may be performed at a weight hourly space velocity (WHSV) inthe range from about 3 to about 200 in some embodiments, and from about6 to about 40 in other embodiments. The reaction may be carried out inthe liquid phase or the gas phase, depending on structure and molecularweight of the olefin(s), by contacting the olefin(s) with the metathesiscatalyst. If the reaction is carried out in the liquid phase, solventsor diluents for the reaction can be used, such as aliphatic saturatedhydrocarbons, e.g., pentanes, hexanes, cyclohexanes, dodecanes, andaromatic hydrocarbons such as benzene and toluene are suitable. If thereaction is carried out in the gaseous phase, diluents such as saturatedaliphatic hydrocarbons, for example, methane, ethane, and/orsubstantially inert gases, such as nitrogen and argon, may be present.For high product yield, the reaction may be conducted in the absence ofsignificant amounts of deactivating materials such as water and oxygen.

Separation of the products from the metathesis reactor may be performedusing any number of combinations of distillation and/or extractivedistillation columns. In some embodiments, such as illustrated in FIG.2, the metathesis reactor effluent may be fed to a series ofdistillation columns including a deethanizer 50, a depropanizer 52, anda depentenizer 56 for separation and recovery of the ethylene recyclefraction 36, the propylene product fraction 38, the C4/C5 recyclefraction 40, and the C5+ purge fraction 42. Other various separationschemes may also be used, and as may be readily envisioned by oneskilled in the art, for separating C2, C3, C4, C5, and heavierhydrocarbon components.

The etherification unit and the decomposition unit may include fixed bedreactor(s), moving bed reactor(s) or other types of reactors. Examplesof various reactors useful in embodiments disclosed herein may includetubular reactors, boiling point reactors, bubble column reactors,traditional fixed bed reactors, catalytic distillation column reactorsystems, pulsed flow reactors, and combinations thereof. One or more ofsuch reactors may be used in parallel flow or series flow, and eachreactor may include one or more reaction zones containing one or moresuitable decomposition catalysts.

In some embodiments, the etherification unit and/or the decompositionunit may include fixed bed reactor(s) that are followed by a separator,such as a distillation column, for separating the reactor effluent intovarious desired fractions, as illustrated in FIG. 3, where like numeralsrepresent like parts. As illustrated, etherification reaction zone 14may include an etherification reactor 60 containing etherificationcatalyst bed 62, the effluent from which may be fed to separator 64 forseparation of tertiary amyl ether fraction 16 from linear pentenefraction 26. Likewise, decomposition reaction zone 18 may include adecomposition reactor 70 containing decomposition catalyst bed 72, theeffluent from which may be fed to separator 74 for separation of alcoholfraction 20 from isopentene fraction 22.

In other embodiments, the etherification unit and/or the decompositionunit may combine the reacting and separating steps in a catalyticdistillation reactor system, as illustrated in FIG. 4, where likenumerals represent like parts. As illustrated, etherification reactionzone 14 may include a catalytic distillation etherification reactionsystem 66 containing etherification catalyst bed 68, for concurrently(i) reacting the alcohol and isopentene to form tertiary amyl alkylethers, (ii) separating the tertiary amyl alkyl ethers from the linearpentenes, (iii) recovering the linear pentene fraction 26 as anoverheads fraction, and (iv) recovering the tertiary amyl alkyl etherfraction 16 as a bottoms fraction. Likewise, decomposition reaction zone18 may include a catalytic distillation decomposition reaction system 76containing decomposition catalyst bed 78, for concurrently (i) crackingthe tertiary amyl alkyl ether to form isopentene and alcohol, (ii)separating the alcohol from the isopentene, (iii) recovering theisopentene fraction 22 as an overheads fraction, and (iv) recovering thealcohol fraction 20 as a bottoms fraction.

Within the scope of this application, the expression “catalyticdistillation reactor system” denotes an apparatus in which the catalyticreaction and the separation of the products take place at leastpartially simultaneously. The apparatus may comprise a conventionalcatalytic distillation column reactor, where the reaction anddistillation are concurrently taking place at boiling point conditions,or a distillation column combined with at least one side reactor, wherethe side reactor may be operated as a liquid phase reactor or a boilingpoint reactor. Divided wall distillation columns, where at least onesection of the divided wall column contains a catalytic distillationstructure, may also be used, and are considered “catalytic distillationreactor systems” herein.

Various combinations of fixed bed reactors upstream (pre-reactor) anddownstream (finishing reactor) of the catalytic distillation reactorsystems are also contemplated for producing propylene according toembodiments disclosed herein.

While separators useful in embodiments disclosed herein are describedabove with respect distillation columns and extractive distillationcolumns, liquid-liquid separations, extractions, or other separationprocesses known to those of skill in the art may also be used. Further,where a single reactor is illustrated, such as for the skeletalisomerization reaction zone, the metathesis reaction zone, theetherification reaction zone, and/or the decomposition reaction zone,embodiments herein also contemplate use of multiple reactors in series,parallel, or a combination thereof.

As described above, embodiments disclosed herein may provide anefficient process for converting mixed pentenes to propylene. The mixedpentenes (linear and isopentenes) are processed through a TAME (tertiaryamyl methyl ether) unit. The isopentenes are reacted with methanol toproduce TAME. However, the linear pentenes remain unconverted in thisunit. The unconverted linear pentenes stream from the TAME unit is anideal feed for metathesis, and is sent to a metathesis reactor toproduce propylene. High purity TAME that is produced from the TAME unitis then cleaved in a TAME decomposition unit, to produce back theisopentenes and methanol. The high purity isopentenes (without linearpentenes) are then processed through a skeletal isomerization unit toconvert the isopentenes to linear pentenes. The effluent from theskeletal isomerization unit, containing a mixture of linear pentenes andunconverted isopentenes, is then sent to the TAME unit. Methanolrecovered during the decomposition of TAME is recycled back to the TAMEunit. By using TAME or other tertiary amyl alkyl ethers as anintermediate, the separation of isopentenes from the linear pentenes issuccessfully accomplished, and the preferred linear pentenes areselectively produced in the skeletal isomerization step. This allows forthe efficient conversion of pentenes to propylene (i.e., linear pentenesto propylene), where the overall scheme allows the use of linearpentenes (with ethylene) to produce propylene without any substantialloss of carbons to undesired side products. Due to the conversion ofisopentenes to linear pentenes, processes disclosed herein may allow forproduction of 36% to 67% higher amounts of propylene as compared tometathesis of mixed pentenes.

While the disclosure includes a limited number of embodiments, thoseskilled in the art, having benefit of this disclosure, will appreciatethat other embodiments may be devised which do not depart from the scopeof the present disclosure. Accordingly, the scope should be limited onlyby the attached claims.

What is claimed:
 1. A process for converting mixed pentenes topropylene, comprising: feeding an alcohol and a hydrocarbon streamcontaining linear pentenes and isopentenes to an etherification reactor;reacting the alcohol and isopentenes in the etherification reactor toconvert at least a portion of the isopentenes to tertiary amyl alkylether; separating the linear pentenes from the tertiary amyl alkyl etherto recover a linear pentene fraction and a tertiary amyl alkyl etherfraction; feeding the tertiary amyl alkyl ether fraction to adecomposition reactor; reacting the tertiary amyl alkyl ether in thedecomposition reactor to convert at least a portion of the tertiary amylalkyl ether to alcohol and isopentenes; separating the alcohol and theisopentenes produced in the decomposition reactor to recover anisopentene fraction and an alcohol fraction; feeding the isopentenefraction to a skeletal isomerization reactor to convert at least aportion of the isopentenes to linear pentenes; recovering an effluentfrom the skeletal isomerization reactor comprising isopentenes andlinear pentenes; recycling the effluent from the skeletal isomerizationreactor to the etherification reactor; feeding ethylene and the linearpentene fraction to a metathesis reactor to convert at least a portionof the linear pentenes and ethylene to propylene.
 2. The process ofclaim 1, further comprising recycling at least a portion of the alcoholfraction to the etherification reactor.
 3. The process of claim 1,wherein the alcohol comprises at least one C1-C4 alcohol.
 4. The processof claim 1, wherein the alcohol comprises methanol.
 5. The process ofclaim 1, further comprising: recovering an effluent from the metathesisreactor comprising ethylene, propylene, C4 olefins, C5 olefins, andheavier hydrocarbon byproducts; separating the effluent from themetathesis reactor to recover an ethylene fraction, a propylenefraction, a mixed C4/C5 fraction, and a heavies purge fraction.
 6. Theprocess of claim 5, further comprising recycling the ethylene fractionto the metathesis reactor.
 7. The process of claim 5, further comprisingrecycling the mixed C4/C5 fraction to the metathesis reactor.
 8. Theprocess of claim 1, wherein the reacting the alcohol and isopentenes andthe separating the linear pentenes from the tertiary amyl alkyl etherare performed concurrently in a catalytic distillation reactor system.9. The process of claim 1, wherein the reacting the tertiary amyl alkylether and the separating the alcohol and the isopentenes are performedconcurrently in a catalytic distillation reactor system.
 10. A systemfor converting mixed pentenes to propylene, the system comprising anetherification reactor for converting an alcohol and isopentenes totertiary amyl alkyl ether; a separator for separating the linearpentenes from the tertiary amyl alkyl ether and to recover a linearpentene fraction and a tertiary amyl alkyl ether fraction; adecomposition reactor for reacting for converting at least a portion ofthe tertiary amyl alkyl ether in the tertiary amyl ether fraction toalcohol and isopentenes; a separator for separating the alcohol and theisopentenes produced in the decomposition reactor and to recover anisopentene fraction and an alcohol fraction; a skeletal isomerizationreactor to convert at least a portion of the isopentenes in theisopentene fraction to linear pentenes; a flow line for recovering aneffluent from the skeletal isomerization reactor comprising isopentenesand linear pentenes and recycling the effluent from the skeletalisomerization reactor to the etherification reactor; a metathesisreactor for reacting at least a portion of the linear pentenes in thelinear pentene fraction with ethylene to form propylene.